This invention relates generally to a method and apparatus for gauging the thickness of a thin layer of material on a surface and more specifically to gauging the thickness of a zirconium barrier layer on the interior surface of zirconium alloy nuclear fuel rods.
The fuel elements used in existing nuclear reactors are found in various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant, nonreactive, heat-conductive container or cladding. The elements may be positioned in a matrix at fixed distances from each other so as to form a fuel assembly. A sufficient number of fuel assemblies are combined to form a nuclear fission assembly, i.e. a reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The cladding on each fuel element serves several purposes. The primary functions are first, to prevent contact and chemical reactions between the nuclear fuel and the coolant and/or the moderator; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant and/or the moderator. Materials commonly used for cladding are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium, certain magnesium alloys and others. If the cladding leaks, or if it should fail, it is possible that the coolant or moderator and the associated systems will be contaminated with long-lived radioactive products to a degree which may interfere with plant operation.
The manufacture and/or the operation of nuclear fuel elements which employ certain metals and alloys as the cladding material may, in some situations, set up the conditions that could give rise to the aforesaid leaks or failure. For example, problems may be caused by mechanical or chemical reactions of the cladding materials under certain conditions. Zirconium and its alloys, under normal circumstances, are well suited for use as nuclear fuel claddings since they have low neutron absorption cross sections. At temperatures below about 750.degree. F. (about 398.degree. C.), such materials are strong, ductile, extremely stable and non-reactive in the presence of demineralized water or steam, which are commonly used as reactor coolants and moderators.
However, where zirconium alloy cladding is used, it has been found expeditious to provide a thin barrier layer between the nuclear fuel material and the cladding material, in order to reduce the possibility of interactions between the fuel material and the cladding. See for example, U.S. Pat. Nos. 4,200,492 and 4,372,817. This barrier layer serves to inhibit damaging interaction between the fuel pellets and the cladding of the nuclear fuel element. Thus, the barrier layer, which may advantageously comprise a low neutron absorption metal such as pure zirconium, serves to protect the substrate from interaction between the fuel pellets and the cladding substrate.
In order to assure quality control, it is desirable to know the thickness of this barrier layer which must be kept uniform throughout. One method for determining the thickness of the layer employs computer controlled metallography and involves time consuming visual measurements. A short piece, about 2 inches long, is cut from each fuel rod during the rod manufacturing process. A small number of these pieces are then taken for testing. A small plastic plug is inserted into one end of the piece and that end is polished and chemically treated so that the barrier layer becomes visible. This end is then placed under a microscope and thickness measurements are visually made at eight places around the circumference of the tube. This procedure involves a significant amount of wasted tube material and, because of the time required in such a procedure, as well as the attendant expense for labor, barrier layer thickness is generally measured on only about 5% of all fuel rods manufactured.
The measurement of coating thickness by X-ray fluorescence is widely known and practiced in different industries, for example to measure the thickness of metal coatings or of paint. See, for example, U.S. Pat. No. 4,208,581. However, such a technique is generally used on planar surfaces only, which can be conveniently accessed so that the size of the measuring apparatus is not critical. For a barrier layer on the inside surface of a nuclear fuel tube having an inside diameter of less than 0.5 inches, heretofore available techniques and apparatus therefor have proved to be incapable of adaptation to such constricted space conditions.